The story revolves around Haruka Nanase, a boy who has always loved to be immersed in water, and to swim in it. Before graduating from elementary school, he participated in a swimming tournament along with his fellow swimming club members, Makoto Tachibana, Nagisa Hazuki, and Rin Matsuoka. After achieving victory, each of the boys went their separate ways.

Time passed, and in the middle of their uneventful high school lives Rin appears and challenges Haruka to a match, showing Haruka his overwhelming power. Not wanting it to end like this, Haruka, gathers together Makoto and Nagisa once again and brings a new member named Rei Ryugazaki to create the Iwatobi High School Swimming Club in order to defeat Rin.

Haruka's kouhai and a first year. He is a straight-forward person and his actions are faster than his words. He admires Haruka's swimming; it's the reason for him to start swimming. He enters Iwatobi High School because Haruka is there.

Rin Matsuoka is Haruka's rival who is also in 11th grade. He had gone abroad to study swimming, but his personality changed immensely by the time he had returned to Japan and for this reason he often perplexes his former teammates.

Rei Ryugasaki is a classmate of Nagisa's in 10th grade. He was originally a member of the track team, but decided to join the swimming club instead. A handsome, intelligent young man who often gets manipulated by Rin for some reason.

Makoto Tachibana is Haruka's inseparable best friend in 11th grade. He understands Haruka's good points, and is often forced to speak as his spokesman. He is very kind and considerate, but is also very easily scared and weak-hearted.